The present invention relates to semiconductor processing chambers, and more particularly, to a rapid thermal processing (RTP) chamber having a barrier for protecting rotational components and other sensitive components of the chamber from the effects of hot process gases. The barrier also provides beneficial features relating to gas flow.
RTP technologies have been developed to increase manufacturing throughput of wafers while minimizing their handling. The types of wafers referred to here include those for ultra-large scale integrated (ULSI) circuits. RTP refers to several different processes, including rapid thermal annealing (RTA), rapid thermal cleaning (RTC), rapid thermal chemical vapor deposition (RTCVD), rapid thermal oxidation (RTO), and rapid thermal nitridation (RTN).
In one RTP process, wafers are loaded into a processing chamber at a temperature of several hundred degrees Celsius (.degree.C.) in a nitrogen (N.sub.2) gas ambient atmosphere. The temperature of the wafer is ramped to a process temperature, usually in the range of about 850.degree. C. to 1200.degree. C. The temperature is raised using a large number of halogen lamps which radiatively heat the wafer. The temperature stabilizes over a pre-set time period. Reactive gases may be introduced before, during or after the temperature ramp. For example, oxygen may be introduced for growth of silicon dioxide (SiO.sub.2).
It is desirable to obtain temperature uniformity in the substrate during processing. Temperature uniformity provides uniform process variables over the substrate (e.g., layer thickness, resistivity, and etch depth) for various process steps including film deposition, oxide growth and etching. In addition, temperature uniformity in the substrate is necessary to prevent thermal stress-induced wafer damage such as warpage, defect generation and slip. This type of damage is caused by thermal gradients which are minimized by temperature uniformity. The wafer often cannot tolerate even small temperature differentials during high temperature processing. For example, if the temperature differential is allowed to rise above 1-2.degree. C./centimeter (cm) at 1200.degree. C., the resulting stress is likely to cause slip in the silicon crystal. The resulting slip planes will destroy any devices through which they pass. To achieve this level of temperature uniformity, reliable real-time, multi-point temperature measurements for closed-loop temperature control are necessary.
One way of achieving temperature uniformity is by rotating the substrate during processing. This removes the temperature dependence along the azimuthal degree-of-freedom. This dependence is removed since, as the axis of the substrate is collinear with the axis of rotation, all points along any annulus of the wafer (at any arbitrary radius) are exposed to the same amount of radiation. By providing a number of pyrometers and a feedback system, even the remaining radial temperature dependence can be removed, and good temperature uniformity achieved and maintained across the entire substrate.
In mechanical rotation systems, such as those sold by Applied Materials, Inc., of Santa Clara, Calif., the substrate support is rotatably mounted on a bearing assembly that is, in turn, coupled to a vacuum-sealed drive assembly. Certain details of such systems are provided in U.S. Pat. No. 5,155,336, entitled "Rapid Thermal Heating Apparatus and Method", issued Oct. 13, 1992, assigned to the assignee of the present invention, and incorporated herein by reference.
For example, FIG. 1A depicts such a system. A wafer 12 is placed on an edge ring 14, which is in turn friction-fit on a cylinder 16. Cylinder 16 sits on a ledge of an upper bearing race 21 which is magnetic. Upper bearing race 21 is disposed within well 39 and revolves, by virtue of a number of ball bearings 22 (only one of which is shown), relative to a lower bearing race 26. Lower bearing race 26 is mounted generally at a chamber bottom 28.
A water-cooled reflector 24 is positioned on chamber bottom 28 as part of the temperature measuring system (details of which are not shown). Reflector 24 reflects radiant energy from heating lamps (not shown) back towards the wafer for efficient heating. Reflector 24 is advantageously used as one side of a reflecting cavity. The other sides of the reflecting cavity are formed by cylinder 16 and wafer 12. The highly reflective properties of reflector 24 allow precise temperature measurements of wafer 12. A magnet 30 is located adjacent the portion of chamber bottom 28 opposite upper magnetic bearing race 21. The magnet is mounted on a motor-driven magnet ring 32.
One problem with rotating substrates during processing is that unstable gas paths, referred to here as "vortices", may be produced. These vortices may occur when process gases traverse from a region over the rotating substrate to a non-rotating region adjacent the substrate, or vice-versa. These vortices may in turn cause other unstable gas paths in unpredictable directions. These unstable gas paths often lead to film nonuniformity.
For example, referring to FIG. 1B, when substrate 12 supported by edge ring 14 on cylinder 16 rotates, the process gases are viscously pulled along substrate 12. When the process gases impinge on a non-rotating portion of the chamber, represented schematically here by element 35, a counter-rotating vortex may be set up by the ensuing unstable gas flow. The steady state reached may be as shown in the figure, with the process gases' path shown by arrow 37 and the counter-rotating vortex indicated by arrows 36. This situation causes an undesirable ring of deposition 38 to occur on window 33 because the hot process gases impinge on window 33 before they can cool, causing condensation. The point at which condensation occurs is often the point at which the two gas paths meet, shown in FIG. 1B as a dotted line.
The above system has other disadvantages. For example, it is commonly seen that the sliding and rolling contact associated with ball bearings leads to particle generation in the processing chamber. This particle generation arises from the contact between the ball bearings and the races as well as from the necessary use of lubrication for the bearing system.
A related disadvantage occurs when gaseous products of the chemical reactions on the wafer are not fully exhausted via a pumping system. Some amount of these gases may escape the pumping system and undesirably flow to regions below the plane of the wafer. For example, a typical silicon deposition may occur by the reaction of trichlorosilane (TCS) and molecular hydrogen (H.sub.2) in a processing region above the wafer. Occasionally, some of the process gases may leak to the region below the wafer due to imperfections in the edge ring supporting the wafer or due to incomplete coverage of the edge ring by the wafer. The leaked gases are then bounded in a cylindrical region which may range from about five to twenty millimeters (mm) high, which is the approximate distance between the wafer and the reflector located beneath the wafer. The temperature at the top boundary of this region (the wafer) may be equal to or greater than 1100.degree. C. The temperature at the bottom boundary of this region (the reflector surface) is typically about 150.degree. C. Under these conditions and this thermal gradient, it is commonly noted that trapped TCS gas is converted to silicon chloride (SiCl.sub.2) and hydrogen chloride (HCl) gases. These gases tend to form undesirable deposits on the cooled reflector surface due to condensation. Such deposits deleteriously affect the temperature measurement of the wafer. Undesirable deposits also occur on the backside of the wafer.
Other regions which may be so affected include the region forming the remainder of the well containing the bearing/race system. Many of the sensitive components relating to rotation may be located in this well. In particular, damage and corrosion may be caused to the bearings and the exterior of the cylinder by the presence of hot gases in these regions.
A further problem associated with present rotational systems concerns heating of those components nearest the wafer. These components, such as upper bearing race 21 (FIG. 1A), receive large amounts of radiant heat from the heating lamp system, and are heated to a substantial fraction of the temperature achieved by wafer 12. Gases which may have adsorbed onto upper bearing race 21 during prior depositions may be outgassed by the heat and thus form the source of potential contaminants.
Thus, it would be useful to provide a processing system that eliminates or reduces the effect of some, if not all, of these problems.